mapbox-gl-transformer.js

function joiner(data, joinRules) {
  return function(tile) {
    if (shouldJoin(joinRules, tile, data)) {
      return joinData(joinRules, tile, data);
    }
  }
}

var countryJoiner(someData, ['country', '=', 'country']);

map.getSource('foo').addPipe(function(tile, callback) {
  callback(countryJoiner(tile)) || tile)
});